In a conventional starter motor for automobiles, for instance the one disclosed in Japanese utility model application laid-open publication (kokai) No. 3-41169, a pinion gear is selectively meshed with the ring gear of an engine, and is rotatively driven by a motor by using a shift lever which rotates according to the displacement of an armature of a magnetic switch.
In a starter of such a structure, the pinion gear is axially slidably fitted on a rotatably supported pinion shaft, and is adapted to move axially in either direction integrally with the push rod which is passed through the pinion gear support shaft.
It sometimes happens that the pinion gear is not able to readily mesh with the ring gear, and buffering means is therefore needed between the armature of the magnetic switch and the push rod. As such buffering means, it has been proposed to use a shift spring consisting of a torsion coil spring which, coaxially mounted on a central pivot shaft for the shift lever, has its one end engaged with the armature and its other end engaged with the shift lever. Thus, when the magnetic switch is activated, but the pinion gear is unable to mesh with the ring gear, and the shift lever is therefore not yet fully rotated as it should, the displacement of the armature is accommodated by the twisting deformation of the shift spring.
However, the shape of the shift lever tends to be complicated because of the need to mount the torsion coil spring thereon, and the efforts required to mount the torsion coil spring may become substantial because of the large initial loading of the torsion coil spring which is required to ensure a sufficiently large force for pushing the push rod or the pinion gear, and also because of the complicated shape of the shift lever. Further, the action of the torsion coil spring tends to be unstable because it is mounted on the pivot shaft of the shift lever, and the loading of the torsion coil spring tends to cause a substantial frictional force acting between the pivot shaft and the torsion coil spring. This frictional force has a tendency to destablize the behavior of the associated contacts of the magnetic switch. To stabilize the action of the torsion coil spring, it is necessary to increase the mounting precision and to design a complicated shape of the shift lever, but it in turn increases the efforts required for assembling the associated parts.